Design and Simulation of a Refractive Quasi-Optical System for Millimetre Wave Imaging

In this paper, a quasi-optical millimeter wave imaging antenna at central frequency of 94 GHz in W-band is designed. The designed imaging system requires resolution less than 30 mm at distance of 5 meters to recognize metal objects. This passive system consists of a combination of a dielectric lens (primary optics) to focus the radiation and a pyramidal horn (secondary optics) with open metal walls to properly direct and couple the focal radiation to the detector. The general parameters of this system are calculated using the geometrical optics method. Then the combination of Gaussian beam method and ABCD ray transfer matrix is used to analyze the lens antenna to find the location and size of the feeding horn. The powerful optical design software ZEMAX was used to design the lens and to verify the design results based on ray tracing. The designed dielectric lens is made of High Density Polyethylene (HDPE) with a diameter of 650 mm. Then, an optimized pyramidal horn with gain of 18.9 dB at center frequency of 94 GHz is designed. The simulation of the horn and investigation of its results performed using the FEKO and CST commercial softwares. The simulation results of the pyramid horn antenna indicate that its combination with dielectric lens is appropriate and the hybrid method (geometrical optics, Gaussian beam and ABCD matrix) to design the quasi-optical system, including the lens and the horn antenna for imaging with considered requirements, is accurate, simple and efficient.